Added SlimuxSendKeys function

This is similar to SlimuxSendCommand, but it is a bit more low level as it doesn't insert carriage return(Enter) automatically and enables sending special keys to the pane(such as ctrl+c for SIGINT).

Also modified SlimuxShellLast so it will prompt for a command if none was sent
before.

I didn't add anything equivalent to the SlimuxShellRun command since if you want to automate a specific key sequence you can just bind it to ':call SlimuxSendKeys' passing the string directly.